The present invention relates to a polymer electrolyte fuel cell using hydrogen, methanol, methanol or dimethyl ether as a fuel and air or oxygen as an oxidant. More specifically, the present invention particularly relates to a composite electrolyte membrane, a catalyst-coated membrane assembly and a membrane-electrode assembly.
Conventional polymer electrolyte fuel cells employing a cation (hydrogen ion) conductive polymer electrolyte generate electricity and heat by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air.
FIG. 17 is a schematic cross sectional view illustrating a basic structure of a unit cell designed to be mounted in a conventional polymer electrolyte fuel cell. FIG. 18 is a schematic cross sectional view illustrating a basic structure of a membrane-electrode assembly designed to be mounted in the unit cell 110 shown in FIG. 17. As shown in FIG. 17, in a membrane-electrode assembly 101, on each surface of a polymer electrolyte membrane 111 capable of selectively transporting hydrogen ions is formed a catalyst layer 112 composed of a hydrogen ion conductive polymer electrolyte and a catalyst body obtained by allowing carbon powders to carry an electrode catalyst (e.g. platinum metal catalyst).
As the polymer electrolyte membrane 111, polymer electrolyte membranes made of perfluorocarbonsulfonic acid such as Nafion (trade name) available from E.I. Du Pont de Nemours & Co. Inc., USA are now widely used.
On the outer surface of each catalyst layer 112 is formed a gas diffusion layer 113 made of, for example, carbon paper treated for water repellency and having gas permeability and electron conductivity. The combination of the catalyst layer 112 and the gas diffusion layer 113 constitutes an electrode 114 (anode or cathode).
A conventional unit cell 110 is composed of a membrane-electrode assembly 101, gaskets 115 and a pair of separators 116. The gaskets 115 are arranged on the outer periphery of the electrodes with the polymer electrolyte membrane 111 sandwiched therebetween so as to prevent the supplied fuel gas and the supplied oxidant gas from leaking out and to prevent them from mixing with each other. The gaskets 115 are usually integrated in advance with the electrodes and the polymer electrolyte membrane 111. In some cases, the combination of the electrodes and the polymer electrolyte membrane 111 and gaskets 115 is called “membrane-electrode assembly”.
On the outer surfaces of the membrane-electrode assembly 101 are placed a pair of separators 116 for mechanically fixing the membrane-electrode assembly 101. On the surface of the separator 116 in contact with the membrane-electrode assembly 101 is formed gas channels 117 for supplying a reaction gas (fuel gas or oxidant gas) to the gas diffusion electrode 114 and removing a gas containing an electrode reaction product and unreacted reaction gas from the reaction site to the outside of the electrodes. Although the gas channels 117 may be formed independently of the separator 116, they are usually formed by providing grooves on the surface of the separator as shown in FIG. 17.
A single unit cell constructed by fixing the membrane-electrode assembly 101 with a pair of separators 116 can produce an electromotive force of about 0.7 to 0.8 V at a practical current density of several tens to several hundreds mA/cm2 when a fuel gas is supplied to the gas channel 117 of one of the separators 116 and an oxidant gas is supplied to the gas channel 117 of the other of the separators 116.
Polymer electrolyte fuel cells, however, are usually required to produce a voltage of several to several hundreds volts when used as power sources. For this reason, in practice, the required number of unit cells are connected in series to give a stack for use.
In order to supply the reaction gas to the gas channel 117, there is required a manifold in which a pipe for supplying the reaction gas is branched into a corresponding number of separators 116 and the branched pipes are directly connected to the gas channels on the separators 116. Particularly, a manifold in which external pipes for supplying the reaction gas are directly connected to the separators 116 is called “external manifold”.
On the other hand, there is another type of manifold called “internal manifold”, which has a simpler structure. An internal manifold is composed of apertures formed in the separators 116 having gas channels 117 formed thereon. The inlet and outlet apertures are connected with the gas channel 117. The reaction gas can be supplied to the gas channel 117 directly from the aperture.
In a polymer electrolyte fuel cell as described above, the electrode reaction occurs on the surface of the catalyst contained in the catalyst layer 112. In the anode-side catalyst layer 112, the reaction represented by the formula (1) occurs. In the cathode-side catalyst layer 112, the reaction represented by the formula (2) occurs. The entire reaction is represented by the formula (3).H2→2H++2e  (1)1/2O2+2H++2e→H2O  (2)H2+1/2O2→H2O  (3)
The reaction given above produces an electromotive force, thus enabling power generation. Due to this electrode reaction, water is produced at the cathode-side catalyst layer 112. In the reaction, H+ generated at the anode-side catalyst layer 112 migrate through the polymer electrolyte membrane 111 to the cathode-side catalyst layer 112. A single H+ migrates along with 5 to 20 water molecules.
The polymer electrolyte membrane 111 exhibits high hydrogen ion conductivity only when it is sufficiently swelled with water. However, because a large amount of water is transferred to the cathode along with H+ that migrates through the polymer electrolyte membrane 111, it is necessary to continually supply water to the polymer electrolyte membrane 111. The water is supplied from the gas channel 17 to the gas diffusion layer in the form of water vapor, and it passes through the cathode or the anode to the polymer electrolyte membrane 111. An excess amount of water, which is not used by the polymer electrolyte membrane 111, in the water generated in the cathode-side catalyst layer 112 is expelled from the gas channel 117 to the outside through the gas diffusion layer 113.
Generally speaking, the polymer electrolyte membrane 111 has the property that its size varies greatly depending on the water impregnated condition of the polymer electrolyte membrane 111. Accordingly, problems arise during the production of a cell or stack in the manufacturing process of a polymer electrolyte fuel cell such as difficulty in alignment of the polymer electrolyte membrane 111 and formation of wrinkles in the polymer electrolyte membrane 111. Further, the polymer electrolyte membrane 111 usually has insufficient mechanical strength, and therefore it might be damaged during the production or operation of a polymer electrolyte fuel cell.
In an attempt to solve the above problems, for example, Japanese Examined Patent Publication No. Hei 05-75835 (Patent Document 1) proposes to use an electrolyte membrane obtained by impregnating a porous film made of polytetrafluoroethylene with a perfluorocarbon polymer having a sulfonic acid group. Japanese Laid-Open Patent Publication No. Hei 06-231779 (Patent Document 2) proposes a polymer electrolyte membrane reinforced by fibrillated perfluorocarbon polymer. Further, Japanese Laid-Open Patent Publication No. 2002-203576 (Patent Document 3) proposes a polymer electrolyte membrane reinforced by a film having apertures with a cross sectional area of not greater than 300,000 nm2 extending vertically in the thickness direction.
However, even with the use of the techniques proposed by Patent Documents 1 to 3, the size change of a water impregnated polymer electrolyte membrane could not be suppressed sufficiently. In addition thereto, the mechanical strength of the polymer electrolyte membrane was not sufficient, either.
In other words, even when the techniques disclosed in Patent Documents 1 to 3 are used, the size of a polymer electrolyte membrane varies depending on the water impregnated condition, the polymer electrolyte membrane is displaced during the production of a cell or stack, wrinkles are caused in the polymer electrolyte membrane, or the polymer electrolyte membrane is damaged during the production or operation of a polymer electrolyte membrane. Accordingly, there still existed room for improvement.
In view of the above problems, an object of the present invention is to provide a polymer electrolyte membrane for a polymer electrolyte fuel cell having excellent size stability and excellent mechanical strength that can sufficiently prevent the size change due to the water impregnated condition, the displacement of the polymer electrolyte membrane and the formation of wrinkles during the production of the polymer electrolyte fuel cell, and can sufficiently prevent damage during the production and operation of the polymer electrolyte fuel cell.
Further, another object of the present invention is to provide a highly reliable polymer electrolyte fuel cell that can sufficiently prevent the displacement of the polymer electrolyte membrane and the degradation resulting from wrinkles and damage, and can exhibit sufficient cell performance for a long period of time.